For example, there is an integrated semiconductor device to be mounted on an optical pickup of a disk drive. The integrated semiconductor device of this type has a semiconductor substrate mounted on which are a semiconductor laser, a signal detection photoreceptor and optical elements such as a prism.
Some integrated semiconductor devices have a monitor photoreceptor providing an APC (Automatic Power Control) function of controlling to make constant the light amount of a laser beam to be emitted from a semiconductor laser.
As types of the monitor photoreceptor, there are a rear monitor type, which receives and detects a portion of a small light amount of a rear side laser beam emitted to a rear side opposite to the front side on which side a front side laser beam is emitted toward the record surface of a disk-shaped recording medium, and a front monitor type, which receives and detects a portion of a front side laser beam emitted toward the record surface of a disk-shaped recording medium.
Generally, a reproduction disk drive only for reproducing an information signal recorded in a disk-shaped recording medium can control a light amount of a laser beam emitted from a semiconductor laser by detecting a small amount of the laser beam with the use of a monitor photoreceptor of the rear monitor type.
However, there are cases where the monitor photoreceptor of the rear monitor type cannot be used for some reproduction disk drives of the type, so-called monolithic two-wavelength laser mounting device type in which a light source, a waveguide, a detector element and the like are fabricated on a single substrate, or for recording/reproducing disk drives also for recording an information signal.
For example, in a recording/reproducing disk drive, when an information signal is recorded in a disk-shaped recording medium, it is necessary to precisely control a large output laser beam emitted from a semiconductor laser. However, it is difficult to precisely control a semiconductor laser beam by using the monitor photoreceptor of the rear monitor type because a ratio between a front side laser beam and a rear side laser beam changes with temperature (temperature drift). In the recording/reproducing disk drive of this type, therefore, the semiconductor laser is controlled by using a monitor photoreceptor of the front monitor type, which is not affected by temperature drift.
An example of a conventional semiconductor integrated device having a monitor photoreceptor of the front monitor type is shown in FIG. 7 (for example, refer to Patent Document No. 1).
A semiconductor integrated device a has a semiconductor substrate disposed in a package not shown, and a semiconductor laser d is mounted on the semiconductor substrate b through a submount c.
A prism e is disposed on the semiconductor substrate b, opposing to the semiconductor laser d. The prism e is formed with a first reflection surface f and a second reflection surface g continuous with the lower side of the first reflection surface f. The first reflection surface f is inclined at an angle of about 45° from an optical axis of a laser beam emitted from the semiconductor laser d, and the second reflection surface g is approximately perpendicular to an optical axis of the laser beam emitted from the semiconductor laser d. The first reflection surface f is a half mirror surface, and the second reflection surface g is a total reflection surface.
Signal detection photoreceptors h, h are disposed on the semiconductor substrate b at lower side positions of the prism e.
A monitor photoreceptor i is disposed on the semiconductor substrate b between the submount c and prism e. The surfaces of the semiconductor substrate b and monitor photoreceptor i are coated with a coating made of, for example, silicon dioxide.
In the semiconductor integrated device a constructed as described above, a laser beam emitted from the semiconductor laser d propagates toward the first reflection surface f of the prism e whereat it is reflected to bend its optical path by 90°. The reflected laser beam enters into an objective lens not shown and is converged on a record surface of a disk-shaped recording medium. The laser beam converged upon the record surface of the disk-shaped recording medium becomes a return light which is made incident upon the first reflection surface f of the prism e via the objective lens and transmits through the first reflection surface f and is received with the signal detection photoreceptors h, h.
Upon reception of the laser beam with the signal detection photoreceptors h, h, for example, an information signal recorded in the disk-shaped recording medium is read.
While a laser beam is emitted from the semiconductor laser d, a portion of the laser beam propagates toward the second reflection surface g of the prism e whereat it is reflected and received with the monitor photoreceptor i disposed between the submount c and prism e.
Upon reception of the laser beam with the monitor photoreceptor i, a light amount of received light is detected, and in accordance with this detection result, the semiconductor laser d is controlled so that the light amount of a laser beam to be emitted from the semiconductor laser becomes constant.
The semiconductor integrated device a described above is, however, associated with some problems such that, since the monitor photoreceptor i is exposed in the air, its sensitivity has a large variation because of a variation in the coated layer on the monitor photoreceptor i, and further, it is likely to be influenced by stray light.
Furthermore, both a laser beam reflected at the record surface of a disk-shaped recording medium and a monitor laser beam enter the inside of the prism e, interference of the laser beams occurs and an information signal detection operation and a control operation for an output of the semiconductor laser d are possibly adversely affected.
In order to suppress the above-described sensitivity variation and the stray light influence, there is a conventional semiconductor integrated device in which a monitor photoreceptor is disposed on the semiconductor substrate at the lower side position of a prism (refer to FIG. 1 of Japanese Patent Application Publication No. 2002-260273).
However, a laser beam received with the monitor photoreceptor of this semiconductor integrated device is a laser beam in the aperture region of an objective lens emitted from a semiconductor laser, resulting in a problem such that a coupling efficiency of an optical system is lowered.
Since both a laser beam reflected at the record surface of a disk-shaped recording medium and a monitor laser beam enter the inside of the prism, the above-described problem of interference of the laser beams is still not solved, similar to the above-described integrated semiconductor device a.
It is therefore a task of an integrated semiconductor device of the present invention to improve the reliability of controlling an output of a laser beam to be emitted from a semiconductor laser, and the like.